One question that I have for religious believers is how they would distinguish between the following:

(1) An all-powerful deity created and guides the universe ultimately towards a good purpose;

and

(2) An all-powerful deity created and guides the universe ultimately towards an evil purpose, but have chosen to maliciously presented himself as benevolent to play a trick on created beings.

I mean, since believers are big on creating conceptual space to make their positions logically POSSIBLE, then it is also possible that God is a Cosmic Trickster who takes pleasure in fooling them.

How could one refuse (2)? Only based upon one's religious beliefs that (1) must be true. The problem is that one's beliefs that (1) must be true could be part of the cosmic joke in scenario (2), and thus there is no real way to differentiate between (1) and (2) for a religious believer.

Aquinas's arguments show that the very notion of an evil god is incoherent. Simply put (not much time), in classical theism God is a purely actual being (n.b. this is the conclusion of rigorous argumentation, not a postulate), and metaphysically, evil is (ultimately) a privation of being. The contradiction is obvious: an evil god would be a purely actual being with a complete privation of being.

But if you're still interested in this sort of argument (i.e. the argument that God may just as likely be an evil being as a good being), the best development of it I've come across can be seen in Stephen Law's God of Eth argument.

In brief, let me spell out the problem with classical theism's approach. If God exists, then by definition everything he does is good, everything. And consequently evil would always be a privation of the good. [Such a view would still not let theists of the hook for the problem then becomes why is there not more goodness in the world rather than why there is evil]. But to say God always does that which is good means nothing except that God does what he does. We simply use the English word "good" to describe all of his actions, and that's it. The word "good" is just a word applied to God. No matter what this God does we are supposed to use the word "good" to describe his actions, whether it's commanding a genocide, sending a tsunami, or an earthquake, a fire or a hurricane.

You see, I do not think the word "good" should apply to that kind of God. By our more civilized notions of goodness those actions are best described by the word "evil." So while God by definition always does what is "good," if what he does is evil then what we have here is a reversal of definitions. By these standards goodness becomes a privation of evil, you see, and so the problem is why there is goodness in the world (i.e., Stephen Law's the God of Eth). Then such a god can indeed exist, a trickster God, an evil one, who, if he exists does evil, even though by definition God always does "good," because at this point the word "good" has lost its meaning. At that point all we can say is that what God does, he does. He does what he does. To attribute the word "good" to that which he does, when he does evil, is reversing the definitions of these words. Words mean what we mean them to mean.

I refuse to play the theistic language game. Theirs is a twisted one, a meaningless one (in this instance) and so an evil God, a trickster God can exist after all. For it no longer means anything to say whatever God does is "good" if what he does by our standards is "evil." He does what he does, so whether we think of him as evil or good no longer matters. They are just words. He does what he does. There is no incoherence with that.